1. Field of the Invention
This invention relates to methods for the formation of halogenated aliphatic thiols and episulfides. More particularly, this invention relates to techniques for producing 2-haloalkanethiols by reacting halogenated olefins with excess hydrogen sulfide and to subsequently converting the 2-haloalkanethiols to the corresponding monomeric episulfide with equimolar amounts of anhydrous ammonia. As such the invention is useful for the preparation of 2-haloalkanethiol pesticides and intermediates, and episulfide monomers.
2. Description of the Prior Art
Episulfides are reactive, desirable monomers. They are useful for the preparation of a variety of polymers ranging from engineering plastics to solvent resistant elastomers. Several synthetic approaches are known for the preparation of episulfides. Nevertheless, no episulfide is commercially manufactured because all the known processes are characterized by either low yield performance or involve expensive intermediates. The present invention unexpectedly improves the yields of two known reactions by using specific, critical process conditions. The two reactions related to the present invention are discussed below:
The first reaction, i.e., the free radical type addition of hydrogen sulfide to vinylic halides, has been known since 1946. However, there is no known process for carrying it out so as to obtain the corresponding .beta.-haloalkanethiol monoadducts. For example, the addition of H.sub.2 S to vinyl chloride ##STR1## WAS DESCRIBED IN U.S. Pat. No. 2,398,480. However the patentee failed to secure high selectivity levels for the formation of the 2-chloroethanethiol monoadduct. The major portion of the product was the diadduct, i.e. di-2-chloroethyl sulfide, commonly known as mustard gas.
In general, an excess of H.sub.2 S might be employed to increase the selectivity of H.sub.2 S/olefin additions to monoadduct products. In the case of vinylic halides, however, the excess of hydrogen sulfide is expected to enter secondary reactions with the 2-haloalkanethiol monoadducts. 2-Haloalkanethiols are generally known to be unstable and highly reactive. For example, C. C. Price and P. F. Kirk describe in the Journal of the American Chemical Society, in volume 75, page 2400 (1953), that 3-chloro-2-butanethiol releases hydrogen chloride on distillation. In the presence of hydrogen chloride an equilibrium is established between .beta.-haloalkanethiols and the corresponding episulfides. This equilibrium is discussed by N. V. Schwartz in the Journal of Organic Chemistry, volume 3, pages 2895 to 2902 (1968). In the case of 2-chloropropanethiol the following reactions are involved: ##STR2##
The episulfide equilibrium product is known to react with hydrogen sulfide. For example, E. M. Meade and F. N. Woodward describe in the Journal of the Chemical Society, pages 1894 and 1895 (1948) that ethylene episulfide reacts with H.sub.2 S to yield ethanedithiol. It is well known in physical chemistry that the consequence of the elimination one product of an equilibrium is the shift of the equilibrium. In the case of the chloroethanethiol-ethylene episulfide equilibrium, the reaction of the episulfide with hydrogen sulfide should consequently lead to the conversion of the 2-chloroalkanethiol to ethanedithiol: ##STR3## In view of the above reaction, an excess of hydrogen sulfide would be expected to have an adverse effect on the yield of 2-haloalkanethiols from vinylic halides. Indeed, to date, no such high yield synthesis is known.
In accordance with the present invention, it has been surprisingly discovered that 2-haloalkanethiols can be synthesized from vinylic halides in high yields by using at least a 3:1 molar excess of the hydrogen sulfide reactant and carrying the addition to a conversion of less than 90% of the vinylic halide reactant.
The dehydrohalogenation of 2-haloalkanethiols by certain bases to yield episulfides is known. The reaction was described in 1939 by W. Coltof in U.S. Pat. No. 2,183,860. According to Coltof the dehydrohalogenation is carried out under slightly alkaline conditions. In the case of weak bases, an excess of the base in water is used.
Coltof states that inorganic nitrogen bases are suitable dehydrohalogenation agents. Such bases would normally include anhydrous ammonia. However, ammonia is known to polymerize the episulfide product of dehydrohalogenation. This polymerization is described by M. Ohta, A. Kondo and R. Ohi in the Nippon Kagaku Zasshi, volume 75, pages 985 and 986 (1954) (see Chemical Abstracts, Vol. 51, page 14668.sup.e, year 1957). Such a polymerization would probably involve the N-H bonds of ammonia as indicated by the reaction scheme, ##STR4## discussed on page 164 of volume IX of Houben-Weyl's Methoden der Organischem Chemie, which was published by Verlag Chemie in Weinheim, W. Germany in 1965.
Consequently, in view of the overall information from the prior art, an episulfide sulfide synthesis using ammonia as a reactant does not seem feasible. It has now been found surprisingly that if an equimolar amount of anhydrous ammonia is used, instead of an excess of the aqueous reagent, a high yield synthesis of episulfides from 2-haloalkanethiols becomes feasible.
The overall concept of the present invention embraces the combination of the above processes plus three known reactions in a cyclic process sequence which converts olefins to episulfide monomers with the net consumption of hydrogen sulfide and oxygen.